The present invention relates to a method and apparatus for measuring characteristics of a photo-conductive semiconductor wafer and, more particularly, to a method and apparatus that measures the resistivity, mobility, and carrier concentration of the semiconductor wafer.
A semi-insulating (SI) semiconductor wafer, such as for example, gallium arsenide (GaAs), is used as a substrate on which circuit elements or other devices are fabricated by epitaxial growth or other methods. After the devices have been fabricated on the substrate, they are tested individually for operational defects which often occur because of the properties of the substrate. Unfortunately, however, the fabrication process represents up to 95% of the cost of the ultimate device. Thus, it would be much more cost effective to test the substrate nondestructively before the fabrication process to avoid unnecessary expense.
Commonly used techniques for testing include standard Hall-effect, photoluminescence, and photo-assisted resistance measurements. To obtain standard Hall-effect measurements, small samples are cut from the substrate. The measurements require a substantial amount of time to prepare the semiconductor sample and ultimately acquire test data. Hence, standard Hall-effect measurement techniques constitute destructive testing that is not really practical. Although photoluminescence can be used for nondestructive testing, measurements must be carried out at very low temperatures. In addition, this technique detects only those imperfections which display a luminescent behavior.
The photo-assisted resistance technique is a nondestructive technique that can be used for measuring semiconductor characteristics. It was first described by R. T. Blunt et al in an article entitled "Dislocation Density and Sheet Resistance Variations Across Semi-Insulating GaAs Wafers" published in the IEEE Transactions on Electron Devices, Vol. ED.-29, No. 7, July 1982 and referred to as dark-spot resistance ("DSR") for measuring sheet resistances across semiconductor wafers. The disadvantage of this technique, however, is that it does not measure true resistivity or provide information related to mobility and carrier concentration. Up to now, there have been no nondestructive, topographic methods or apparatus to measure the true resistivity, mobility and carrier concentration characteristics.
Accordingly, there is a need for a method and apparatus for nondestructively measuring the resistivity, carrier mobility and carrier concentration of a semi-insulating semiconductor wafer.